It appears there's been a change in the wording on the
custom's site . It now states that you need to have evidence of residency when the container/goods arrive in NZ rather than when you first enter the country. Compare the wording with Beth's (Ebianca) post above. It's a very recent change, but one for the better.
When No Customs Charges Apply
Eligibility
Your household or other related effects (excluding motor vehicles, boats and aircraft) will qualify for concessionary entry, i.e. free of duty and exempt GST, for a reasonable time after you arrive in New Zealand, provided you can meet all of the following conditions.
Conditions
* You have arrived and on the date the effects are imported, hold a document authorising residence in New Zealand; and
* You have resided or lived outside New Zealand for the whole of the 21 months before the date of your arrival in New Zealand; and
* You have owned and used the goods before the date of your departure for New Zealand; and
* The goods are for your own personal use and not intended for any other person or persons or for gift, sale or exchange.